This invention relates to the continuous casting of liquid metals and is especially useful in the continuous casting of steel.
Continuous casting has been a commercial success in the world for about two decades, enjoying both technical and economic advantages over ingot casting. Continuous casting is extremely well suited for computerized control. However, previous attempts at computerized control have been complex and expensive, such as is taught in Adams U.S. Pat. No. 3,358,743 assigned on its face to Bunker Ramo.
In the continuous casting of steel a furnace is tapped into a refractory lined ladle equipped with a bottom pour mechanism such as a stopper rod as shown in Bruderer et al U.S. Pat. No. 3,946,795 or a slide gate system as shown in Shapland U.S. Pat. No. 3,352,465. The hot metal is poured from the ladle into an intermediate pouring vessel known as a tundish, which is equipped with one or more bottom pour nozzles, depending on the number of casting strands or pouring streams desired. Usually each nozzle is located above a continuous casting mold, although with a very wide mold, such as is used in a slab caster, two or more pouring streams may deliver molten metal to a single casting mold. Each mold is open ended and has water cooled mold walls.
Both the ladle and the tundish may be equipped with a slide gate pouring system or a stopper rod pouring system or a combination. However, this invention is principally intended for use in a continuous casting system where the ladle only is equipped with nozzle control including a slide gate closure member or a stopper rod closure member, and the tundish outlet is plugged with asbestos or some other convenient plug system which is pulled from the bottom to initiate pouring into a continuous casting mold. In practice a molten steel filled ladle is positioned with the nozzle above the tundish. The nozzle is opened on the ladle to permit steel flow into the tundish. After molten steel has reached a predetermined level, the tundish nozzle or nozzles are opened, allowing the steel to teem into the mold. The mold is temporarily closed at its bottom with a dummy bar head including a chill plate and chill pins and/or scrap, which, along with the water cooling of the mold, causes the steel to become partially solidified along the mold sidewalls and the bottom of the casting. The casting removal system is then actuated to withdraw the dummy bar. As the casting, which is now connected to the dummy bar, leaves the mold, it is supported by slide plates or rolls and guides. Water sprays are directed onto its surface to increase the solidification of the casting skin. Usually a casting follows a curved path, after which it passes through straightening rolls. By the time the casting reaches the straightening rolls, it either has only a very small liquid core or is totally solidified. Excessively hot steel in the mold and insufficient cooling in and beneath the mold can cause re-melting of the solidified skin, resulting in a break out.
The flow of the steel into the mold is principally controlled by the cross-sectional area of the tundish nozzle, and to a small extent, by the ferrostatic head or steel height in the tundish. Since the depth of the tundish is a known factor, and the nozzle sizes are known, the casting rate thus varies in a predetermined range. Casting speed heretofore has been subject to variation because the height of the molten metal in the tundish varies during the pouring of a heat, and because the ladle and tundish nozzle areas increase due to erosion caused by the flow of steel through the nozzles. Therefor, controlling the flow rate within a portion of this range will insure that the caster is operated at a higher casting rate at the usual break out rate or at a usual casting speed at a reduced break out rate.